Development of additively manufactured diamond-coated scaffolds for orthopaedic applications

With the increasing frequency of hard tissue replacement and fracture treatment, novel materials are required to fabricate long-term implants. Commonly, metal scaffolds are used to fabricate orthopaedic implants, on which soft tissue and/or bone grows around the metal surface. Although metals have proven to be effective, significant problems remain in terms of the osseointegration of the material and bacterial susceptibility. In the biomedical field, carbon is becoming an element of choice due to its biocompatibility with the human body. More specifically, diamond, the sp3 form of carbon, can potentially be used to enhance the biocompatibility of standard implant materials. In this thesis, polycrystalline diamond (PCD) and nanodiamond (ND) are used to coat titanium scaffolds using multiple seeding and coating methods. After physical and chemical characterisation of the diamond, the scaffolds are examined under in vitro conditions. The additive manufacturing method is utilised to incorporate patient specificity into the underlying material. This PhD thesis focuses on the development of diamond coatings to provide an improved bio-interface for additively manufactured titanium structures. The first stage of the project includes a detailed characterisation and understanding of the material surface/interface. The second stage of the research is to understand the fabricated materials under in vitro conditions. The stages will investigate the surface morphology, chemistry, roughness, wettability, coating thickness, cell attachment and proliferation, and bacterial viability. Chemical vapour deposition (CVD) is employed to investigate the coating capacity of PCD. To demonstrate potential orthopaedic applications, selective laser melted titanium alloys (Ti-6Al-4V; SLM-Ti) were fabricated using a layer-by-layer approach while maintaining a high strength-to-weight ratio of the underlying material. The SLM-Ti substrates were seeded in a slurry of NDs to ensure that the PCD could nucleate from the carbon gas mixture. A full material assessment was undertaken, showing PCD film thicknesses of 2-3 µm on the SLM-Ti substrates. The in vitro study found a higher density of Chinese Hamster Ovarian (CHO) cells on PCD-coated substrates than on uncoated SLM-Ti substrates. This result suggests that the surface morphology, chemistry and roughness are key influences on mammalian cell adhesion and proliferation. The PCD-coated scaffolds also inhibited bacterial (S. aureus) attachment four times more effectively than uncoated scaffolds. Furthermore, to indirectly investigate the in vivo bone activity on the scaffolds, the thickness of the apatite layer formed on the substrates was measured after 14 days of incubation in simulated body fluid (SBF). The thicknesses of the apatite-like mineral layer were 315 nm on as-fabricated SLM-Ti, 210 nm on polished SLM-Ti and 1.34 µm on PCD-coated SLM-Ti. These findings reveal the significant advantages of PCD-coated SLM-Ti scaffolds in terms of biocompatibility, inhibition of bacterial attachment and enhanced osseointegration capabilities. The use of PCD coatings on SLM-Ti scaffolds shows the potential for coating planar substrates and that the surface chemistry of diamond plays an important role in mediating mammalian and bacterial cell responses.  <br><br> In the next step, PCD is used to coat complex, non-planar substrates after the optimisation of the CVD growth conditions. The quality and coverage of the PCD coatings was assessed on SLM-Ti structures of various shapes, showing enhanced uniformity. A novel method for coating three-dimensional structures is established using a so-called 'Faraday cage' fabricated from SLM-Ti. The presence of the Faraday cage reduces the plasma enhancement or 'hot-spots' compared to traditional cage-free CVD methods. The coating of three-dimensional structures can be useful for several applications in the medical, optical and mechanical industry. <br><br> The following study aims to elucidate the response of bone cells on PCD coated SLM-Ti scaffolds. One of the major problems with the current orthopaedic implants is the lack of osseointegration, which is the structural connection between bone and the surface of the artificial material. To determine the response of bone cells to PCD, cell adhesion and proliferation of Rat Calvariae Primary Osteoblasts (OBs) were measured on PCD coated scaffolds after 1, 3 and 7 days of incubation. Additionally, focal adhesion (vinculin) and elongation of actin filaments of OBs on these substrates were quantified using fluorescent microscopy. The responses of the OBs were more favourable towards the PCD coated scaffolds than the uncoated scaffolds. The proliferation and elongation of the cell are based on the coverage of the diamond coating. These results demonstrate that the surface chemistry, wettability and surface roughness of diamond coated substrates all contribute towards the improved OB cell response. <br><br> Despite PCD being an excellent form of diamond coating, there are limitations with the CVD-grown thin film in terms of the coverage that it can provide. Chemical vapour deposition uses a line-of-sight technique. As a result, internal surfaces are difficult to coat. Hence, in the final part of the research, NDs were employed for coating using a more facile approach, dip coating. Initially, drop casting, spin coating and dip coating were used to assess the feasibility of ND coatings. The results show that dip coating provided the best coverage and functionality for up-scaling. To identify the optimal conditions for a uniform coating, several concentrations of the ND suspension were trialled: 0.075% w/v, 0.75% w/v and 7.5% w/v. The coating with 7.5% w/v ND suspension showed greater human dermal fibroblast and OB cell density, which were 32% and 29% more than those of uncoated SLM-Ti surfaces after 3 days of incubation, respectively. In addition to the improved cell growth, the ND coating also inhibited S. aureus bacterial colonisation by 88% more than the uncoated SLM-Ti substrate. This study shows that dip coating of ND is an effective way to create antifouling SLM-Ti scaffolds. <br><br> Finally, the goal of this PhD is to demonstrate the fabrication of a more biocompatible material using diamond coatings compared to un-coated additively manufactured titanium scaffolds. The coated scaffolds show the capacity to potentially improve osseointegration while limiting the risk of bacterial infections. As such, the developed materials could improve the functionality and lifetime of medical implants.